The present invention relates to a manufacturing method of optical modules. More specifically, the invention relates to a novel manufacturing method of a pigtail-type optical module in which an electronic circuit including an optical element (photoelectric conversion element or electrooptical conversion element) and an optical fiber for introducing an optical signal to the optical element or extracting an optical signal from the optical element are coupled to form a single unit.
In the pigtail-type optical module, an electronic circuit including an optical element (light-emitting element and/or photodetecting element) and an optical fiber for introducing an optical signal to the optical element or extracting an optical single from it are coupled to form a signal unit. That is, the pigtail-type optical module is mounted on a circuit board together with electronic circuits etc. appropriate for the purpose, and serves as a component interfacing between the electronic circuits and an optical signal transmission line.
FIG. 9 shows a structure and a manufacturing method of a conventional pigtail-type optical module which is a light transmission module having a laser diode (LD) as a light source.
As shown in FIG. 9, in the conventional pigtail-type optical module, a circuit board 12 on which an integrated circuit 11 is mounted and a sub-assembly 13 including an optical fiber 13a and an optical element coupled to each other to form a single assembly are incorporated in a metal package 14 to constitute a signal unit.
The optical module having the above structure is manufactured by the following process. First, the sub-assembly 13 is produced by combining, with a sheath 13c, a sub-package 13d containing the optical element and the optical fiber 13a to which a ferrule 13b is attached. Then, the sub-assembly 13 is attached to the metal package 14 having the circuit board 12, and the sub-assembly 13 and the circuit board 12 are electrically connected to each other by soldering or bonding. Finally, the metal package 14 is sealed by a cover, to become the complete optical module. The optical coupling efficiency between the optical fiber 13a and the optical element is adjusted by aligning their optical axes at the time of producing the sub-assembly 13.
In actual production of the above type of optical modules, a separately supplied sub-assembly is used which is a single component including an optical element and an optical fiber or a receptacle portion. However, even where sub-assemblies according to the same standard are used, finally produced optical modules unavoidably have a variation in optical coupling efficiency due to variations of the light output power of the optical element, dimensions of the parts used, etc. On the other hand, each of the optical communication systems etc. provide a standard defining the upper and lower limits of the input and output light powers of an optical module used therein. For those reasons, in optical module manufacturing processes, optical modules are assembled using selected sub-assemblies. The drive current etc. of each module is closely adjusted for the characteristics of the sub-assembly used. Therefore, the yield of sub-assemblies and the total productivity is low, and it is difficult to lower the price of shipped optical modules and to realize mass-production of optical modules.
As a countermeasure, it has been proposed to apply, to the optical module production process, the transfer molding technique that is employed in the packaging of mass-production-type integrated circuits. FIG. 10 shows an example of a structure of a sub-assembly to be used in manufacturing optical modules using the transfer molding technique.
As shown in FIG. 10, according to the transfer molding technique, a sub-assembly 22 including a receptacle portion 21 for receiving an optical connector connected to the end portion of an optical fiber and a light-receiving portion or light-emitting portion 25 consisting of an optical element 23, lens 24, etc. are preliminarily wired to a lead frame (not shown) on which an electronic circuit is mounted. The wired assembly is then molded with a transfer mold resin. After the molding, unnecessary portions of the lead frame are removed and the outer leads are shaped.
In the above process according to the transfer molding technique, a metal mold containing the parts needs to be heated to 150.degree.-200.degree. C. in the molding step. Therefore, mainly from the heat resistance of the optical fiber, it is concluded that the above process cannot be applied to the production of the pigtail-type optical module which includes even the optical fiber as a member of the single unit. Further, in another manufacturing process in which the optical fiber is attached after the transfer molding step, there is no method for adjusting the optical coupling efficiency between the optical fiber and the optical element, so that the production yield is not stable.